Drizzle a large pan lightly with olive oil, and add as many bread slices as fit in one layer. Turn the heat to medium, and cook until golden brown and toasted underneath, about 4 minutes. Flip, and toast the second side, about 1 minute more, then set toast aside on a plate. Repeat with the remaining bread, adding more oil as needed. Remove from heat and rub toasts with the halves of the garlic clove, and sprinkle with salt.
Increase the heat to medium/medium-high, and add enough oil to recoat the pan well. Heat the oil until hot, and add the Brussels sprouts. season well with salt and freshly ground black pepper, and let them cook undisturbed until well browned underneath, 3 to 5 minutes. Flip, and let the second side brown well underneath, 2 to 4 minutes more. Stir once or twice, and repeat this browning-and-flipping process until the Brussels have charred spots all over. Remove from the heat, and squeeze the juice of half your lemon over the top. Taste, and season with more salt and pepper if needed.
Spread each slice of bread thickly with ricotta. Heap the ricotta with the charred Brussels sprouts. Finish with another drizzle of oil, squeeze of lemon juice, salt and red-pepper flakes, to taste. Scatter with hazelnuts.